Fix skip back behaviour when jumpin to start of book (#201)

After last changes, pressing the jump back button when less than 30 seconds of the book had been played caused the pointer to jump to the beginning, but not the audiobook itself. This was due to the AudioBookPlayer not jumping back if there was no actual point to jump to.

This is now fixed by computing how much of the book we have listened, and if it's less than 30 seconds, we jump to the beginning of the book. Speeding up the book causes the jump to beginning happen seemingly from further away than 30 seconds, but only a few seconds further away. It shouldn't be too noticeable for users.

NOTE: If the jump to the beginning happens from another chapter than the first one, the audiobook pauses.

Author: natlibfi-burger

simplified-viewer-audiobook/src/main/java/org/librarysimplified/viewer/audiobook/EkirjaPlayerFragment.kt

@@ -597,6 +597,12 @@ class EkirjaPlayerFragment : Fragment(), AudioManager.OnAudioFocusChangeListener
    */
   private fun skipBack() {
     updatePlayerPosition = true
+    //Skipping back doesn't work correctly if the book hasn't been played for 30 seconds
+    //So in that special case, we skip to te beginning of the book
+    // If we jump from another chapter to the beginning, the player pauses
+    if(needsToSkipToBeginning()) {
+      return player.playAtBookStart()
+    }
     return player.skipBack()
   }
 
@@ -1139,7 +1145,7 @@ class EkirjaPlayerFragment : Fragment(), AudioManager.OnAudioFocusChangeListener
 
     //Current time is accurate presentation of the time
     //That current chapter has played
-    val currTime = TimeUnit.SECONDS.toMillis(lastStartedPlayerPosition
+    val currentTime = TimeUnit.SECONDS.toMillis(lastStartedPlayerPosition
       .plus(
         (TimeUnit.MILLISECONDS.toSeconds(
           offsetMilliseconds
@@ -1151,19 +1157,19 @@ class EkirjaPlayerFragment : Fragment(), AudioManager.OnAudioFocusChangeListener
     playerRemainingBookTime.text =
       PlayerTimeStrings.hourMinuteTextFromRemainingTime(
         requireContext(),
-        (getCurrentAudiobookRemainingDuration(spineElement) - currTime)
+        (getCurrentAudiobookRemainingDuration(spineElement) - currentTime)
       )
 
     // The visual presentation of how much of the chapter is left
     this.playerTimeMaximum.text =
       PlayerTimeStrings.hourMinuteSecondTextFromDurationOptional(spineElement.duration
-        ?.minus(currTime))
+        ?.minus(currentTime))
     this.playerTimeMaximum.contentDescription =
       this.playerTimeRemainingSpokenOptional(offsetMilliseconds, spineElement.duration)
 
     // The visual presentation of how much of the chapter is done
     this.playerTimeCurrent.text =
-      PlayerTimeStrings.hourMinuteSecondTextFromMilliseconds(currTime)
+      PlayerTimeStrings.hourMinuteSecondTextFromMilliseconds(currentTime)
     this.playerTimeCurrent.contentDescription =
       this.playerTimeCurrentSpoken(offsetMilliseconds)
 
@@ -1339,4 +1345,28 @@ class EkirjaPlayerFragment : Fragment(), AudioManager.OnAudioFocusChangeListener
 
     playerService.updatePlayerInfo(this.playerInfoModel)
   }
+
+  /**
+   * Returns true if we have played the book for less than 30 seconds
+   */
+  private fun needsToSkipToBeginning() : Boolean {
+    //Length of whole book
+    val wholeBook = book.spine.sumOf { it.duration?.millis ?: 0L }
+    //How much of current chapter has passed
+    val currentChapterPassed = TimeUnit.SECONDS.toMillis(lastStartedPlayerPosition
+      .plus(
+        (TimeUnit.MILLISECONDS.toSeconds(
+          this.playerPositionCurrentOffset
+        ).minus(lastStartedPlayerPosition
+        ))))
+
+    //How much of book is remaining
+    val bookRemaining = getCurrentAudiobookRemainingDuration(this.playerPositionCurrentSpine!!).minus(currentChapterPassed)
+
+    // Return true if the difference is less than 30 seconds,
+    // which indicates the need to jump to the beginning of the book
+    // NOTE: On faster play rates, the jump to start of book happens also from a little further
+    // due to the increased speed
+    return wholeBook.minus(bookRemaining) < 30000
+  }
 }